A modern application specific integrated circuit (ASIC) must meet very stringent design and performance specifications. One example of an ASIC is a circuit element referred to as a serializer/deserializer (SERDES). As its name implies, a SERDES converts a parallel bit stream to a high speed serial bit stream, transmits it across a channel, then the serial bit stream is converted back to a parallel bit stream. A typical SERDES is organized into blocks of transmitters and receivers having digital to analog conversion (DAC) functionality and analog to digital conversion (ADC) functionality. Normally, the receivers and transmitters operate on differential signals. Differential signals are those that are represented by two complementary signals on different conductors, with the term “differential” representing the difference between the two complementary signals. All differential signals also have what is referred to as a “common mode,” which represents the average of the two differential signals. High-speed differential signaling offers many advantages, such as low noise and low power while providing a robust and high-speed data transmission.
Typically, it is desirable that high-speed differential input/output circuits (also referred to as input/output buffers, receiver/transmitter circuits, or receiver/driver circuits) use some form of differential and common mode termination (e.g., a resistive load) to match the differential impedance of the transmission medium (or channel). The transmission medium (e.g., printed-circuit board traces, transmission lines, backplanes, a differential wire pair, or cables) couples the output circuit to the input circuit and provides a path along which the intended information travels.
Because the receivers only respond to differential voltages, common mode modulation is generally rejected by the receivers. However, common mode signals may create certain problems with differential signaling systems. For example, the common mode signals, if not terminated, may consume a large portion of a receiver's finite common mode voltage range or, if the common mode signal is driven into resonance, exceed the common mode range of the receiver. Common mode signals may interfere with or degrade the communication of the desired information.
FIG. 1 is a schematic diagram illustrating a conventional transmit driver and receiver termination network. The network 1 comprises a transmit (TX) portion 2 and a receive (RX) portion 4 connected by a transmission medium 17 and 18. The transmit portion 2 comprises a transmit driver 6 and a transmit driver 8. The transmit drivers 6 and 8 deliver a differential transmit signal to resistors 7 and 9, respectively. The DC current through each resistor is given by Icm=Vcm/Rt, where Icm is the common mode transmit current, Vcm is the common mode voltage of the differential signals, and Rt is the value of the resistors 7 and 9.
The signal INP is the positive transmit signal, also referred to as an input signal, provided to a receiver 13. The signal INN is the negative transmit signal, also referred to as an input signal, provided to a receiver 14. The terms “positive” and “negative” are relative because the signals INP and INN represent the components of a differential signal that exist around the common mode voltage, Vcm. The signals INP and INN may have the same polarity or may have different polarity. A receiver termination resistor 11 and a receiver termination resistor 12 terminate the INP and INN signals, respectively, to circuit ground. The values of the resistors 11 and 12 are chosen to represent the impedance of the transmission medium 17 and 18, respectively, which is shown in dotted line to indicate that the transmit portion 2 is separated from the receive portion 4 by a distance. In an embodiment, the value of each resistor 11 and 12 is nominally 50 ohms.
FIG. 2 is a diagram illustrating an output signal of the transmitter drivers of FIG. 1. The output signal from the transmit drivers 6 and 8 is one where the common mode voltage is Vcm and is measured at the receiver inputs as:
                    VCM        =                              INN            +            INP                    2                                    Eq        .                                  ⁢        1            
A non-zero common mode voltage, for the network 1 (FIG. 1), generates DC current and DC power. Because hundreds of transmit and receiver channels are integrated into a typical integrated circuit, it is desirable to minimize this power. For the example transmit and receiver circuit shown in FIG. 1, Vcm is typically 0.25 volts resulting in 10 mWatts of power assuming a 1 volt supply.
FIG. 3 is a schematic diagram illustrating a conventional transmit driver and receiver termination network that minimizes DC power. The network 21 comprises a transmit (TX) portion 22 and a receive (RX) portion 24 connected by a transmission medium 37 and 38. The transmit portion 22 comprises a transmit driver 26 and a transmit driver 28. The transmit drivers 26 and 28 deliver a differential transmit signal to resistors 27 and 29, respectively. The DC current through each resistor 27 and 29 is shown as Icm=0 to reflect that there is ideally zero (0) DC current in the resistors 27 and 29 because, as will be described below, there is ideally zero (0) DC current flowing in the resistors 31 and 32.
The signal INP is provided to a receiver 33. The signal INN is provided to a receiver 34. A receiver termination resistor 31 and a receiver termination resistor 32 terminate the INP and INN signals, respectively, to a voltage source 35, which is illustrated as a DC voltage source at the common mode voltage, Vcm. The values of the resistors 31 and 32 are chosen to represent the impedance of the transmission medium 37 and 38, respectively, which is shown in dotted line to indicate that the transmit portion 22 is separated from the receive portion 24 by a distance. In an embodiment, the value of each resistor 31 and 32 is nominally 50 ohms. The network 21 solves the excessive power problem of the network 1 by disconnecting the receiver termination from ground and connecting it to a voltage source having the value, Vcm. With the receiver common node at Vcm, the DC current through the receiver termination resistors 31 and 32 is zero and the DC power is zero. Unfortunately, a voltage source 35 is difficult to implement in a cost-effective manner.
FIG. 4 is a schematic diagram illustrating a conventional transmit driver and receiver termination network that implements the network 21 of FIG. 3. The network 41 comprises a transmit (TX) portion 42 and a receive (RX) portion 44 connected by a transmission medium 57 and 58. The transmit portion 42 comprises a transmit driver 46 and a transmit driver 48. The transmit drivers 46 and 48 deliver a differential transmit signal to resistors 47 and 49, respectively. The DC current through each resistor 47 and 49 is shown as Icm=0 to reflect that there is ideally zero (0) DC current in the resistors 51 and 52 because, as will be described below, there is ideally zero (0) DC current flowing in the resistors 51 and 52.
The signal INP is provided to a receiver 53. The signal INN is provided to a receiver 54. A receiver termination resistor 51 and a receiver termination resistor 52 terminate the INP and INN signals, respectively, to a capacitor 55. The capacitor 55 represents a simple implementation of the voltage source 35 of FIG. 3. The values of the resistors 51 and 52 are chosen to represent the impedance of the transmission medium 57 and 58, respectively, which is shown in dotted line to indicate that the transmit portion 42 is separated from the receive portion 44 by a distance. In an embodiment, the value of each resistor 51 and 52 is nominally 50 ohms Replacing the Vcm voltage source 35 (FIG. 3) with the capacitor 55 (common mode capacitance (Ccm)) also solves the excessive power problem, but at the expense of common mode return loss.
Receiver common mode return loss is given by:
                    CM_RT        =                              -            20                    ·                      log            ⁡                          (                                                                    Z                    receiver                                    -                  25                                                                      Z                    receiver                                    +                  25                                            )                                                          Eq        .                                  ⁢        2            
FIG. 5 is a diagram 60 illustrating a typical receiver return loss mask (receiver return loss). To pass a typical common mode return loss mask, the impedance of the capacitor 55 at 10 MHz should be less than 50 ohms. This dictates a capacitor in excess of 225 pf, which is difficult to economically integrate into the termination network 41 due to large size. Further, integrating such a large capacitor also causes leakage and introduces a variety of manufacturing difficulties.
Accordingly, a receiver termination network that overcomes these shortcomings is needed.